Multi carrier methods are used both in wired and wireless communication systems. Examples for wired systems are ADSL (Asymmetric Digital Subscriber Line), VDSL (Very high rate Digital Subscriber Line), or more generally xDSL. Examples of wireless multi-carrier systems are IEEE 802.11a, IEEE 802.11g och Hyperlan II, which are all based on OFDM (Orthogonal Frequency Division Multiplex) technology.
Typically a multi carrier system sends coded information from a sender to a receiver on a set of N carriers (sometimes also called sub-carriers), where N is an integer, typically in the interval 64-4096, depending on the system.
Since the radio spectrum suitable for wireless communication is a limited and shared resource, the spectrum assigned to any given system intended to serve multiple users distributed over a possibly large service area must be reused in different sub-areas, or cells, of the total service area, in order to provide sufficient coverage and capacity. Depending on the multiple access technique used, the distance between different cells, or base stations, assigned to the same portion of the available spectrum may vary. In order for a mobile station in an arbitrary location in the service area to determine which cell or which base station to connect to, the base stations must transmit information that can be used by the mobile stations to identify the different base stations that are in range of the mobile station.
In multi-carrier systems the available bandwidth is divided into a number of sub-channels. These subchannels can be used for carrying different types of payload and/or control information. For example, in the base stations of cellular networks, one or more of the available sub-channels can be reserved for cell-specific broadcast information, which will enable the mobile stations both to detect the presence of and to synchronize to different base stations in order to retrieve cell-specific information. Similar to conventional frequency reuse, the sub-channels reserved for this purpose can be reused in different cells that are sufficiently far away from each other propagation wise.
To perform cell search and retrieve cell-specific information, at least to some extent, the set of physical resources allocated for this purpose should be known by the mobile stations in advance. In addition, to support mobile stations belonging to different generations, this minimum allocation should be static during the entire lifetime of the network. Hence, the allocation strategy for sub-channels conveying cell-specific transmissions is of fundamental importance when designing a system.
In multi-carrier systems with many sub-channels, clearly many different allocation strategies can be formed to define an aggregate physical information channel. However, a good strategy should take into account diversity and receiver complexity:                Diversity. In case of channel delay spread, the frequency response of the overall channel will not be flat. Thus, the different sub-channels will face different attenuation and phase shifts. Hence, in order for the mobile stations to be able to detect and synchronize even in the case of frequency selective fading, multiple sub-bands that are sufficiently spaced in frequency to minimize the risk of contemporaneous bad transmission conditions should be allocated.        Receiver complexity. When mobile stations are not active, that is, not conducting traffic, they will sleep in order to conserve battery power. However, to maintain synchronization to the serving base station and check for new base stations potentially offering a better connection, the mobile stations must periodically “wake up” and perform measurements. Thus, to help the mobile stations to preserve battery capacity, a good allocation strategy should support low-complexity implementations for detecting the sub-channels that carry cell-specific information.        